Project Summary/Abstract Through research and training activities described in this K08 proposal, the PI will acquire the skills necessary to become an independent clinical science researcher in the comprehensive study of alcohol co-use behaviors. Aside from tobacco, alcohol and marijuana are the most commonly used substances nationwide, and the most frequently co-used. With the rapid legalization of marijuana in the United States, it is likely that rates of co-use will continue to rise, particularly among young adults who currently report the highest rates of use. Despite the combined legislative changes and rising public acceptance of marijuana, little is known about how simultaneous use of alcohol and marijuana (SAM) impacts alcohol behaviors. Additionally, necessities of laboratory research (e.g. lower potency marijuana, controlled drinking contexts) present challenges in studying the effects of SAM use. The purpose of the proposed study is to examine whether SAM use leads to increased alcohol consumption and consequences by utilizing ambulatory assessment (AA) methodology. AA involves the integration of assessment methodologies (e.g. self-report, behavioral, physiological) to study individuals in their natural environment. The proposed study would be the first to employ novel behavioral assessments of disinhibition and motor impairment and biological measures of alcohol use (transdermal alcohol concentration: TAC) to study the impact of SAM use on alcohol consumption at the event level and consequences at the daily level. The study involves original data collection from young adult regular alcohol and marijuana users (N=80) who will complete 4 weeks of AA data collection while wearing alcohol biosensors to assess TAC. A combination of self-initiated and random reports of alcohol and marijuana use, subjective intoxication, and craving; paired with behavioral assessment of disinhibition and motor impairment (gait and balance) will be collected. The PI will collaborate with an impressive mentorship team carrying extensive experience in four areas of training: 1) behavioral and pharmacological effects of alcohol and marijuana co-use (Jane Metrik, PhD); 2) ambulatory assessment (Robert Miranda, PhD; Timothy Trull, PhD); 3) alcohol biosensor technology (Nancy Barnett, PhD); and 4) advanced statistical analyses (Kristina Jackson, PhD). The proposed 5-year training plan will prepare the PI for a career as a clinical scientist with expertise in the study of alcohol co-use behaviors; with the ultimate goal of conducting mixed-method designs to capture behavior in controlled laboratory conditions and the natural environment. Research and training will occur at the Center for Alcohol and Addiction Studies (CAAS) at Brown University. CAAS is an interdisciplinary center devoted to research and training in the study of addiction. The proposed research will advance our understanding of mechanisms by which SAM use impacts alcohol use and consequences by building on knowledge gained from controlled laboratory research. Results will inform policy, prevention, and intervention efforts to reduce the negative outcomes associated with simultaneous use of alcohol and marijuana.